1. Field of the Invention
This invention relates to a liquid crystal display device in which a common electrode signal Vcom of the display panel of the display device is easily adjusted by a manufacture of the liquid crystal display device, and an adjusting method thereof.
2. Description of the Related Art
In recent years, liquid crystal panels have been widely used for TV sets or cellular phones. FIG. 8 shows an equivalent circuit diagram of a pixel of a liquid crystal panel according to the conventional art. Such pixels are disposed in a matrix of m rows and n columns. A gate signal line 50 and a drain signal line 51 are formed on an insulating substrate (not shown), intersecting each other. A pixel selecting thin film transistor 52 connected to both the lines 50 and 51 is provided in a periphery of the intersection of those lines. A common electrode signal Vcom is applied to a common electrode of a liquid crystal 53.
Furthermore, a storage capacitance element 55 for storing a voltage of a display electrode 54 for one field is provided. A terminal 56 on one side of the storage capacitance element 55 is connected to a source 52s of the pixel selecting TFT 52, and an electrode 57 on another side of the storage capacitance element 55 is applied with an electric potential common to the pixels.
As shown in FIG. 9, when a gate scanning signal Vg of high level is applied to the gate signal line 50, the pixel selecting TFT 52 turns on so that a video signal Sig is transmitted from the drain signal line 51 to the display electrode 54 and stored in the storage capacitance element 55. The video signal Sig applied to the display electrode 54 is applied to the liquid crystal 53, and the liquid crystal 53 is aligned in response to the signal voltage.
If a DC component is constantly applied to the liquid crystal 53, a degrading phenomenon such as burn-in occurs. Therefore, as shown in FIG. 10, a line inversion driving method in which the polarity of the video signal Sig is reversed every 1H period is employed. In this method, it is necessary to set the video signal Sig so as to change symmetrically with respect to the common electrode signal Vcom in order to avoid generating of the DC component.
However, in parctice, the voltage applied to the liquid crystal 53 is lowered by ΔV as shown in FIGS. 9 and 10. This occurs since a parasitic capacitance 60 is formed between the gate and the source 52s of the pixel selecting TFT 52 so that the source 11s is lowered by ΔV by capacitance coupling, when the gate scanning signal Vg changes from a high level to a low level. This causes the DC component of ΔV to be applied to the liquid crystal 53. Therefore, the common electrode signal Vcom also needs to be lowered by ΔV (to Vcom′ in FIG. 10). However, since ΔV varies for each of manufactured liquid crystal panels, the common electrode signal Vcom needs to be adjusted for each of the liquid crystal panels.
FIG. 11 is a flowchart showing a production flow from manufacturing of the liquid crystal panel by a liquid crystal panel manufacturer to shipping the product to the market by an assembling manufacturer. On the side of the liquid crystal panel manufacturer, liquid crystal panels are manufactured (step 1), inspected (step 2), and shipped to the assembling manufacturer (step 3). The assembling manufacturer, which receives the liquid crystal panels, detects and sets an optimum common electrode signal Vcom for each of the liquid crystal panels (step 4). It has been known as a detecting method of the optimum value of the common electrode signal Vcom to scan the common electrode signal Vcom while monitoring brightness of the liquid crystal panel and set the common electrode signal Vcom when the brightness is at the minimum as the optimum common electrode signal Vcom.
Each of the liquid crystal panels, which is set with the optimum common electrode signal Vcom, is assembled in a set such as a TV set or a cellular phone (step 5), and then shipped to the market (step 6).
As described above, since the assembling manufacturer needs to detect the optimum value of the common electrode signal Vcom for the liquid crystal panel and set it therein, the assembling manufacturer is burdened with many manufacturing steps.
Therefore, the invention is directed to a liquid crystal display device in which the assembling manufacturer using the liquid crystal panel can easily set the optimum value of the common electrode signal Vcom, and an adjusting method of the liquid crystal display device.